Check valves are well known for a variety of applications where it is desirable to prevent fluid flow through a fluid line when certain conditions are present. Check valves generally include a circular valve head which is biased, such as by a spring, into a closed position against an annular valve seat. An O-ring or other type of annular resilient gasket can be carried by the valve head or valve seat and provide a fluid-tight seal when these elements are together. The check valve is located within the fluid line with the valve head biased in the upstream direction. Fluid in the fluid line is directed into the inlet side of the check valve against the valve head. Fluid above the cracking force of the spring causes the valve head to move away from the valve seat and allow fluid to flow through the valve to the outlet side of the valve. The flow generally increases as the valve head moves away from the valve seat toward the fully open position. One typical check-valve application is to prevent flow through a fluid line when the pressure of the fluid drops below a certain level, for example during the shut-down of a fluid system. The check valve closes to prevent fluid leakage when the pressure drops below this threshold.
Certain aircraft turbine engines require check valves to control fuel flow from a manifold to the nozzle tip. For example, DuBell, et al., U.S. Pat. No. 5,078,324, which is owned by Parker-Hannifin Corporation, the assignee of the present invention, shows and describes a simple check valve located in the nozzle head of a fuel nozzle assembly for a gas turbine engine. The check valve includes a spring-biased circular valve head at the end of a rod-like poppet valve. The poppet valve is biased by a spring to urge the valve head against the valve seat. The check valve controls the fuel flow through a single fuel conduit in the nozzle stem to the nozzle tip. The check valve remains closed such at low or no-flow conditions (for example at engine shut-down) to prevent fuel leakage through the nozzle. The valve head moves away from the valve seat at higher pressures to allow fuel to flow to the spray nozzle tip and into the combustor chamber for ignition. The nozzle head is typically located exterior to the combustor chamber of the engine to protect the check valve from the extreme operating temperatures.
While the above type of check valve is relatively simple to assemble and install, current check valve designs for aircraft engines include a cylindrical poppet valve which is closely received within the valve body. The valve body is turned-in at the upstream end to form a valve seat, and the poppet valve includes a circular upstream end wall which defines a valve head and is designed to mate with the valve seat. One or more radial through-holes are provided in a single circumferential band around the poppet valve near the valve head to direct fuel passing around the valve head into the inner cavity or chamber of the poppet valve, where the fuel then flows through the outlet opening of the check valve. When the fuel pressure exceeds the cracking force of the valve, the fuel flows between the valve seat and valve head, around the exterior of the poppet valve, and then radially inward through the through-holes and axially out of the chamber. This type of check valve, sometimes referred to as an "over-balance" valve, provides increased force at the open position than at the closed position, which results in increased stroke and reduced pressure drop as compared to the prior designs described above.
Certain fuel nozzles ("dual orifice" nozzles) also include a flow divider valve located in the nozzle head, downstream from the check valve. The flow divider valve divides the fuel flowing from the check valve into a primary and secondary fuel path. The primary and secondary fuel paths are then directed by separate conduits to primary and secondary spray orifices in the fuel nozzle. Such a spray nozzle having primary and secondary fuel conduits to primary and secondary spray orifices is shown in Mains, U.S. Pat. No. 5,570,580, which is also owned by the assignee of the present invention, as well as in Helmrich, U.S. Pat. No. 3,684,186 and Bradley, U.S. Pat. No. 4,600,151. A typical flow divider valve for gas turbine engines is shown and described in Burke, et al., U.S. Pat. No. 4,570,668, which is also owned by the assignee of the present invention. Of course, the above dual orifice fuel nozzle is only an example of one application for a check valve, as check valves are also necessary in some aircraft engines with only a single fuel path to the spray nozzle ("simplex" nozzles), as well as in other aircraft and non-aircraft applications. A known spray nozzle having a single fuel path is shown for example in Simmons et al., U.S. Pat. No. 3,980,233.
One challenge in the aircraft industry has been to provide a check valve for a fuel nozzle assembly which operates over a broad flow range. At low flow levels (low fuel pressure), the check valve should have an adequate stroke to prevent particles from becoming entrapped between the valve head and valve seat. This can impede the sealing capabilities of the check valve and require frequent maintenance and/or replacement of the valve. One technique which has been used is to restrict the flow path across the check valve. The radial through-holes in the poppet valve for example can be made smaller to increase the pressure against the upstream surface of the poppet valve, thereby moving the poppet valve a greater amount at lower fuel flows. However, restricting the flow across the check valve necessarily increases the pressure drop across the valve. This can be unwanted in certain applications, particularly applications which also have high fluid flows (high fuel pressure) under certain operating conditions (e.g., full-throttle). A significant pressure drop can limit fuel flow to the engine and thereby effect engine efficiency. Applicant believes that heretofore the design of the check valve has required somewhat of a compromise between engine efficiency and the stroke of the poppet valve typically finding a balance that does not optimize either of these properties and therefore does not maximize engine performance over a broad flow range.
Applicant further believes that prior check valve designs have often required considerable space in the nozzle assembly and have been fairly heavy. This can increase the material cost of the aircraft and the cost of fuel for operating the aircraft. The check valves are also often complex and time-consuming to assemble, and to install in the nozzle assembly, all of which can add to the over-all cost of manufacturing and maintaining the fuel system, and hence the aircraft. Applicant therefore believes there is a demand in the industry for an improved check valve for a fuel nozzle assembly which operates over a broad flow range, is compact in design, has a reduced weight and is easy to assemble and install, in order to meet the more demanding applications currently required in the aircraft industry.